Implantable and insertable medical devices are commonly provided with one or more coatings which may serve a wide variety of functions including, for example, imparting biocompatibility, enabling drug delivery, providing lubricity, and so forth.
As one specific example (among many others), coronary stents such as those commercially available from Boston Scientific Corp. (TAXUS and PROMUS), Johnson & Johnson (CYPHER), and others are frequently prescribed for maintaining blood vessel patency. These products are based on metallic expandable stents with polymer coatings, which release antiproliferative therapeutic agents at a controlled rate and total dose for preventing restenosis of the blood vessel. One such device is schematically illustrated, for example, in FIGS. 1A and 1B. FIG. 1A is a schematic perspective view of a stent 100 which contains a number of interconnected struts 100s. FIG. 1B is a cross-section taken along line b-b of strut 100s of stent 100 of FIG. 1A, and shows a stainless steel strut substrate 110 and a therapeutic-agent-containing polymeric coating 120, which encapsulates the entire stent strut substrate 110, covering the luminal surface 110l (i.e., the inner, blood-contacting surface), the abluminal surface 110a (i.e., the outer, vessel-wall-contacting surface), and side 110s surfaces thereof.
One recent development in metallic implants is to base such implants on metallic alloys that degrade in vivo. For example as described in U.S. Patent App. Pub. No. 2002/0004060 A1, entitled “Metallic implant which is degradable in vivo,” implants may be formed from pure metals or metal alloys whose main constituent is selected from alkali metals, alkaline earth metals, iron, and zinc. Particularly preferred are metals and metal alloys containing magnesium, iron or zinc as a main constituent and one or more additional constituents selected from the following: alkali metals such as Li, alkaline-earth metals such as Ca and Mg, transition metals such as Mn, Co, Ni, Cr, Cu, Cd, Zr, Ag, Au, Pd, Pt, Re, Fe and Zn, Lanthanides such as La and Ce, Group IIIa metals such as Al, and Group IVa elements such as C, Si, Sn and Pb. Of these, magnesium alloys have high corrosion (biodegradation) rates, particularly in the presence of NaCl, which is found in vivo.